This invention relates to a method and lubricating oil composition for providing improved friction reducing properties.
There has been considerable effort in recent years to develop lubricating oil compositions which will reduce friction in engines and particularly automotive engines. This effort is based on the need to improve the fuel economy of such engines which operate on petroleum fuel, a product having a declining source of supply and an escalating price. It is known that high engine friction causes significant energy loss and thus one way to improve fuel economy is to reduce such friction.
Known ways to solve the problem of energy losses due to high friction e.g., in crankcase motor oils include the use of synthetic ester base oils which are expensive and the use of insoluble molybdenum sulfides which have the disadvantage of giving the oil composition a black or hazy appearance.
Another approach to reduce energy losses by the use of a lubricating oil composition involves a composition containing an additive combination of an oil soluble friction reducing ester component derived from esterification of a polycarboxylic acid with a glycol and a zinc dihydrocarbyl dithiophosphate. This type of composition is disclosed in U.S. Pat. No. 4,105,571 which notes that while a lubricating composition containing the additive package of such ester and zinc thiophosphate components provides excellent anti-friction and anti-wear properties, it presents compatability and stability problems. The problem is solved by adding a selected ashless dispersant containing a high molecular weight aliphatic hydrocarbon oil solubilizing group attached thereto to the composition, and preferably predispersing either the ester and/or zinc thiophosphate components prior to their addition to the remainder of the composition. Such ashless dispersants may contain hydroxyl groups as noted in the patent.
While the above approaches all provide reduced engine friction and consequently improved fuel economy, there is always the need and desire for further reductions in energy losses due to friction or otherwise since even small reductions per engine can result in a rather large savings in fuel, particularly when considering the total number of engines in use.